In a building having wood structures including beams and pillars, etc., intentionally exposed to be utilized as a design, a joint structure that enables wood materials to be firmly joined together while preventing a joint body from being exposed to the surface is adopted. In this joint structure, for example, a hole expansion is formed in an inner wall of a hole provided in a butt end of each of to-be-joined wood materials to be joined, the respective butt ends are butted against each other, a joint body such as a threaded rod, etc., is housed in and across the holes, and an adhesive agent is filled in the respective holes and cured to join the wood materials together in such a manner that the joint body that is integrated with the cured adhesive agent, is prevented from coming out.
As such a device for forming a hole expansion in an inner wall of a hole of a wood member, there is a “device for forming an expansion in a hole” disclosed in Patent Literature 1. This device includes an insertion pipe to be inserted in a joint hole, a rotor which passes through the insertion pipe and is pivotally supported rotatably, and to which an electric motor is connected, a cutting blade attached to the rotor by a central shaft turnably between inside and outside of the insertion pipe, a tip end bearing that pivotally supports the rotor at the tip end of the rotor, a suction pipe to discharge chips produced inside the joint hole, and a cutting blade operation tool that turns the cutting blade in at least a range from an angle at which the cutting blade is housed inside the joint hole to an angle at which the cutting blade becomes perpendicular to the rotor, and the device can form, as a hole, expansion, a substantially spherical space to be filled with an adhesive agent in the inner wall of the joint hole formed in the joining wood material.